I had thought about using those two holes, but it's a lot of force to put through two M5 bolts. I don't think they'd take it.

The bolts dont take the shearing load though, the bolts act in tension only, the shear reaction force is provided by the friction between the shim and dropout. Mill it flat and bolt&glue the shims on, you could even mill some grooves into it and matching ones into the shims. You could even wrap it in epoxy and s-glass or CF.

I am not a mechanical engineer (but I still think its a better idea than welding).

or drill out the dropouts to 12mm or larger, and mill the shim to fit into that, you only really need a 4-5mm hole for the QR, so thats quite a solid bit of metal, you could even raise the rear end by 2-3mm to allow for even more metal in the dropout. Might need a custom disk from hope to suit though.

I just went out to measure the exact dimensions of a Rohloff axle on Mrs MTG's Kona Big Unit and looking at the sliding dropouts on that, combined with your idea, thisisnotaspoon, I think I may have another solution.

The Kona dropouts fit within the width of the frame, more or less as I need to do on the tandem.
They also put the axle below the level of the actual frame dropout.
Like this.

It's hard to describe in words, but I'll have ago;
Cut a new dropout from 13mm steel.
Machine 8mm of it away from all except the axle slot in the frame. It is now a 5mm plate with a 8mm thick boss on one side. This will locate it and support the weight.
Bolt it to the frame using those two M5 holes. This will also locate it and keep it in place with the wheel removed.
Cut a 10mm slot below the 8mm thick boss to take the Rohloff axle. This will raise the rear end slightly to match the longer suspension forks I will be using. If I get it right, it also means I can use a 180mm disc adapter with a 200mm disc.

In fact, thinking about this a bit more, I could probably fabricate this in steel myself.
I'll make another cardboard model to show you what I mean.

Imagine the cardboard is 5mm steel and the bolt head is a 10mm steel bar welded to it from the back.
Now imagine the slot is the correct shape to take a Rohloff OEM1 axle plate, like this;

This could well be the cheapest, easiest, strongest option.
No risk of weakening the frame.
No burnt paint.
If I make my plate big enough to reach the caliper bolt holes, then it acts as a shim for the caliper adapter and transfers the torque reaction of the Rohloff hub to the caliper mounting. In fact, it will even reduce the torque load as it will be further away from the centre.

Aye - if you are moving the dropout, then you would have to make two. I was thinking of a way to resolve some of the issues with your original idea - those to do with trying to weld on thin (5mm) plate.

You can see the holes in the ply where the axle would originally have been. I'll be putting 10mm bolts through there to take most of the load.
The gear hanger is going to need cutting as well. I'll incorporate a new one in to the right hand dropout as I will need somewhere to attach a tensioner.
That's a 160mm brake mount on there. If I've got my measurements right, it should now line up with a 200mm disc.
The frame is sold as suitable for 26" or 700c wheels and a rigid fork, it's even got two sets of rim brake bosses.
I'll be using 26" wheels, so as the front will be raised a bit by a suspension fork, I reckon raising the rear end by 20mm should put the BB heights and head angle somewhere near right.

That's a 160mm adapter with a 200mm disc, so the frame is raised 20mm.
Having measured the BB height, I have now bought a 140mm adapter and will make another dropout to raise it another 10mm.

I know it looks rough now, just hacked out of aluminium, but I think once I've tapped the axle holes to 10mm and fitted the bolts to clamp it to the frame, it will be strong enough for a solo test ride.
Then when I'm sure it works, I'll cut them out of 5mm steel.

MTG you should speak Ben @ Kinectics cycles. He modified a Rolhoff hub for TJ to fit in his tandem.
If I remember right it involved spacing out the the hub rather than spacing in the frame. The advantage would be that you could fit a standard tandem hub if you wanted to at a later date.

Anga, the load will be carried by the M10 bolts where the axle should be. The M6 caliper bolts and M5 rack bolts are just for location.

Bikepawl, I think Ben replied on another thread and I've seen what Jeremy has done, we are in contact via Facebook.
He put a 10mm spacer on one side of the hub, which means he had to dish the wheel to suit and make a 10mm spacer for the caliper.
I think my solution is neater. No extra spacers for the caliper or gear hanger and any 135mm hub wheel, Rohloff or derailleur, will drop straight in.

One disadvantage of the above design, apart from lifting the back end of the bike a little too much, is that the chain is now too close to the chain stay.
That's a 140mm adapter, so I think I'll go back to a 160mm, which will drop the frame, or raise the axle, by 10mm.

HoratioHufnagel, I am aware that Rohloff hubs are not as reliable as Rohloff themselves would like everyone to believe.
Their warranty service is pretty good though, it seems to be unlimited by age or mileage and free, apart from the £12 postage costs via the LBS.

Welding is not required--you can bolt at a couple of places.
Think of a shoe or glove--insert the dropout into the adapter and bolt it into place. The adapter cups the dropout. The axle goes through the original dropout--I am assuming you can bolt it into place.

I see what you mean now, anga.
Yes, that is one idea I had considered. If I wanted to keep the axle in the original position then I would have to do something like that as the Rohloff axle only extends 6mm beyond the locknuts, so the weight would be almost entirely on the dropouts I have made and they would need some sort of strong support.

A bit of a quickly bodged mock up, but something like this, except I would use bolts or welding, not molegrips.

Trying out different dropouts and fork lengths, I'm getting BB heights of between 340mm and 390mm.
Of the few tandem frame manufacturers who post their geometry on line, it looks like 270mm is about average.Ventana is the one exception, at 343mm/361mm, before sag.
Tandem Jeremy's is about 370mm as well, allowing for sag, so I'll stick with what I'm doing for now, as it's easier, although if it does feel too tall, I may yet go for your idea, anga.

What I would do in this situation is to make one 10mm thicker righthand dropout to replace the existing gear hanger. An exact copy secured in the same way, except thicker. The 135mm wheel would then sit 5mm offset to the left in the existing left hand dropout. The 5mm offset to the left would make no discernible difference to the bike (i've seen plenty of standard bikes worse than this), but you could re-dish the wheel if you were really bothered about it. Everything will then fit and work as normal, and loads will be fed into the frame as they were intended to be.

Took your picture and made minor changes. The dark areas (some of which may be mechanically impossible or not useful) support the dropout and transfer force. You can also use a larger bolt(s) in one of the shaded areas for support. Welding may not be needed.

Remember this thread ?
We've done a few miles on the 5mm aluminium dropouts now and they haven't bent or broken yet.
They are still a bit rough looking, and I'm still not sure whether to just tidy them up a bit, make them in 5mm steel, or make them in 6mm aluminium and spread the frame 2mm.

I'll be swapping to a simple pull up tensioner. It's only a 13t rear sprocket, so I want to get as many teeth engaged as I can.

That's a 200mm disc, I'll be fitting a 205mm to line up with the caliper better.
Caliper, Rohloff and pannier rack all clearing each other nicely.

In short, most people stick with In Phase, because it's the "standard" set up.
Of those who try Out Of Phase, it's about half and half either prefer it or give up and go back to In Phase.
Some go the full 90 degrees, but that's not practical for an off road tandem.
Putting the stronger rider ahead is the most common arrangement and it seems to be better for us, it definitely feels smoother.

First long ride today and it brought a problem with the drop outs to light.
You can see in the pictures above that they are attached to the frame with M10 bolts threaded in to the drop out.
Not surprisingly, the bolt on the chain side, where there is most strain, pulled out of the 5mm thick aluminium, allowing the drop out to move slightly, resulting in the tyre touching the chainstay.
I've modified it now by using countersunk bolts and dome nuts.